One concept for increasing the capacity of optical storage media is to use holographic data storage. In this case the surface or the whole volume of the holographic storage medium is used for storing information, not just a few layers as for conventional optical storage media. Furthermore data can be stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc.
In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded in the form of the data pages. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
The performance of a holographic data storage system can be improved by implementing a phase mask in the optical setup. Such a phase mask introduces random or pseudo random phase shifts to the reference beam and/or the object beam, which results in a better shift selectivity of the system. In addition, high beam intensities inside the holographic storage medium are avoided as the focus diameter of a focused beam is expanded. This technique is known as correlation multiplexing or speckle multiplexing. One disadvantage of this method is that a phase mask with exactly the same pattern must be implemented in all compatible holographic data storage systems to enable readout of a holographic storage medium which was written with another system. The reason is a general aspect of holography: The reference beam for readout (also called probe beam) must have the same properties as the reference beam used during writing. This includes the phase distribution of the reference beam.
To circumvent the above disadvantage, a hologram multiplexing method with a speckled reference beam generated by the photorefractive beam-fanning effect has been proposed by M. Bunsen et al.: “Hologram multiplexing method with photorefractive beam-fanning speckle”, Advanced Optical and Quantum Memories and Computing, Proc. of the SPIE, Vol. 5362 (2004), pp. 128-135. In this method, a bulk photorefractive crystal takes the role of generating various speckle fields as well as storing holograms. The speckle field of a reference beam used for holographic recording is generated by the photorefractive beam-fanning effect in the storage crystal itself. A special alignment of the crystal axes relative to the reference beam and the object beam is needed to ensure generation of speckle fields for the reference beam while avoiding a excessively large beam-fanning effect of the object beam.